Origin and evolution of the Paleozoic Cabo Ortegal ultramafic-mafic complex (NW Spain): UPb, RbSr and PbPb isotope data

Abstract

UPb and RbSr dating of various minerals from the mafic-ultramafic Cabo Ortegal complex reveal the occurrence of high-grade metamorphism and mantle melting between 406 and 383 Ma. This event reflects deep subduction of oceanic lithosphere underneath a continental plate margin, including melting of detrital material and carbonates. Magmatism lead to the injection of locally garnet-bearing pyroxenite layers into the ultramafic rocks. It also caused the emplacement of carbonate-rich garnet-clinopyroxene rocks and pegmatites into peridotites and mafic granulites. The upper time limit is events is defined by the crystallization of 406 ± 4 (2 σ) Ma zircon in carbonate-rich garnet pyroxenites, and the lower limit is given by 383 ± 1 Ma old rutile from a garnet pyroxenite, and by 383 ± Ma titanite that crystallized in a carbonate-rich layer. All other zircon, monazite and titanite analyses lie between these two age limits defining an average age of 388 ± 1 Ma. These small age differences observed between the different minerals substantiate that the UPb chronometer in zircon, monazite and titanite behaved as a closed system in the garnet-clinopyroxene stability field (∼800°C and 1.35–1.65 GPa). The new geochronological data neither confirm the occurrence of a 480 or 420 Ma high-grade metamorphic event, as suggested earlier. In consequence, the Cabo Ortegal complex most likely was produced by a single subduction event (406–383 Ma), which was immediately followed by obduction, and incorporation of the complex into the orogenic belt of NW Iberia. Zircon dating of a plagiogranite in the Ophiolitic Series that tectonically underlies the complex substantiates basaltic crust formation at 472 ± 3 Ma, in association with melting of Precambrian detrital material. This Ordovician magmatic event probably occurred coevally with formation of the protoliths from which the mafic granulites and eclogites of the Cabo Ortegal complex were produced by subduction ∼70 m.y. later. On the other hand, emplacement of the exceptionally well-developed pyroxenite layers in the Cabo Ortegal complex appears to be exclusively a result of peridotite re-melting during earliest Devonian subduction at 406–383 Ma.